ACT5880
Rev 2, 03-Sep-13
TSC ADC
The ACT5880 has a shared SAR ADC with a state-
machine for deploying actions to do resistive touch
panel measurement as well as normal ADC
conversion. The features and performance of the
circuit are optimized for applications with hand-held
size touch panel and related housekeeping. The
shared ADC core has 12 or 8 bits resolutions,
assures that it fits both the position measurement
and the contact evaluation.
Table 7:
The Key Feature of the TSC ADC.
PARAMETERS
Resolution (bits)
Input Impedance
12 or 8
11MΩ
Input Capacitance
32pF
2.5V
The touch screen measurement circuit employs an
analogue front end and a touch detection circuit
with acquisition management, makes the circuit a
full function TSC (i.e., the touch screen controller).
Refer to Figure 10 for the top level block diagram of
the TSC ADC circuit.
Force Driving Voltage and ADC Reference
Maximum Force Driving Current
18mA
AUX0, AUX1, AUX2
Full Scale Input
2.5V
Voltage
AUX3(VD)
6.024V
Figure 10:
The TSC ADC Block Diagram.
Table 8:
The Touch Screen Interface Connections.
XPOS XNEG YPOS YNEG CHANNEL DESCRIPTION
PEN
Hi-Z
Hi-Z GND
2.5V GND
Standby
ADC Hi-Z
0
1
2
3
Y-coordinate
X-coordinate
Z2-coordinate
Z1-coordinate
2.5V GND ADC
ADC GND 2.5V
Hi-Z
Hi-Z
Hi-Z GND 2.5V ADC
Notes:
PEN: The pen touch detection connects to this pin;
2.5V: The 2.5V reference buffer output;
ADC: One of the ADC channel input.
The analogue front end is designed to provide force
driving bias and signal routes for 4-wire type
resistive touch panel and the auxiliary inputs for
common purpose ADC. Both the touch panel
measurement and the common purpose ADC
conversion are organized to manipulate with
operation to same sets of registers, the state
machine controls scanning to get each signal
converted in a sequential frame or doing a single
channel conversion, as it is programmed. Refer to
Acquisition Configuration for details. And refer to
Table 7 for the key specifications.
The respective force and sense to touch panel is
shown in Figure 11.
The resistors lay in vertical and horizontal represent
the equivalent resistances of the transparent films
split by the touch spot, the resistors lay oblique is
the equivalent of the films contacting in the touch
spot, the paths in bold are the force current goes
through. Considering the current into the ADC input
is neglecting comparing the force current and the
even gradient slope of potential in the area between
two bias bar, the channel 0 and channel 1 read
exact the electrical equivalent coordinates in the
axes (the electrical equivalent coordinates then are
scaled to graphic user interface coordinates by the
application software). Giving the 3 equivalent
resistors in the force paths for the channel 2 or
channel 3 connections make the Z axis, the channel
4-wire Touch Screen Interface
The TSC ADC has following internal connections
for touch screen force and sense, as showed in
Table 8.
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